The design trend in modern data communication systems is towards ever higher frequencies. This is also evident in the analog design, in which the maximum achievable sampling frequency is increasing further as a result of the use of ever smaller semiconductor structures. In order to allow an analog signal to be produced correctly at the output of a D/A converter, the sampling rate must be at least twice the maximum signal frequency. The suppression of the repetition spectra which are created by the sampling process is normally ensured by an analog filter.
Many data communication appliances also use digital signal processing and operate with an equivalent baseband signal in the form of an in-phase (I) and quadrature (Q) component. Two D/A converters (D/A=digital-to-analog) are therefore required for conversion of the equivalent baseband signal to the analog domain before the digital modulation onto the carrier. It is important for this modulation process to retain the orthogonality of the I and Q components.
Further applications of two D/A converters occur in audio processing. Two D/A converters are likewise required for stereophonics. Three or more D/A converters are required, in a corresponding form, for multiple-channel audio processing.
Two (or more) separate D/A converter paths may in this case be highly resource-intensive in terms of the chip area. With regard to the current that is drawn as well, two (or more) separate D/A converter paths make a major contribution to the total current drawn. The different characteristics of two D/A converters may be found to be problematic, resulting from discrepancies (mismatches) in the processing and manufacture of the semiconductor module. The different behaviour may, for example, result in distortion in relatively high-quality modulation processes. One arrangement which uses this principle for the A/D converter paths (A/D=analog-to-digital) is disclosed in U.S. Patent Application Publication 2003/0215027 A1.
FIG. 9 shows a block diagram of a D/A converter apparatus for multiple-channel audio processing. For each audio channel (right, left, center), the D/A converter apparatus has a D/A converter path in each case formed from a D/A converter 901 and a filter 903, connected downstream from the respective D/A converter 901, for suppression of repetition spectra.
FIG. 8 shows a block diagram of a D/A converter apparatus for the equivalent baseband modulation, in which the in-phase component (I component) and the quadrature component (Q component) are separately converted from digital to analog form and filtered. A separate D/A converter 801 and a corresponding filter 803 for suppression of the repetition spectra are therefore required for each of the digital signals (I component, Q component).
In comparison to a simple embodiment, the use of a plurality of D/A converters results in much more chip area being required. The current drawn when two or more D/A converters are used is also a multiple of when only one is used. The identical requirements for the separate channels result in very stringent requirements for the matching of the characteristics during manufacture and semiconductor processing.
Furthermore, the digital-to-analog converters provided for each of the signal processing paths must have the same response. This is particularly necessary in UMTS modules, in order to ensure that the I channel and the Q channel have characteristics which are as identical as possible. Different channel characteristics can lead to signal attenuation, to signal delays and to distortion. These effects are undesirable. A further aim is to eliminate further negative interference effects, such as common-load interference, by formation of the difference between the signals involved. In this case as well, it is desirable for the signal paths involved to have the same response.
There is therefore a general aim with integrated modules to achieve the same signal-path response by an exactly identical design of the paths involved and physical proximity on the chip. Even if it were possible to position the paths involved close together, although this would be good for the matching of the components involved, it would, however, not in fact be optimal. An exactly identical design has the disadvantage that the components must be duplicated. This consumes a larger area, and thus increases the production costs.
FIG. 7 shows one possible D/A converter principle, which is based on a resistance divider using switching transistors which pass the desired voltage value to the outside. One D/A converter is provided in each case, both for the I component and for the Q component. Each of the converters has a driver 701 for production of the reference voltage for the I component and Q component, respectively. Furthermore, physically identical resistance chains 703 are provided. Taps 705 are provided between successive resistors in the respective resistance chains 703, so that an analog value which corresponds to a digital value is output when one of the switches 707 is closed. The arrangement as illustrated in FIG. 7 for digital-to-analog conversion of the I path and Q path is used, for example, in UMTS modules. If the electrical characteristics of the D/A converters illustrated in FIG. 7 differ, then this results in an undesirable mismatch between the I component and the Q component.